Reducing-noise with dual-mode antenna



Aug. 12, 1969 TINGYE Ll REDUCING NOISE WITH DUAL-MODE ANTENNA 2 Sheets-Sheet 1 Filed Aug. 30, 1967 mozmdiou lNl/ENTOR TTORNEY Aug. 12, 1969 TINGYE u 3,461,453

, REDUCING NOISE WITH DUAL-MODE ANTENNA I Filed Aug. 50, 1967 2 Sheets$heet 2 4s FMIJLTIPUERWNTEGRATOR SQUARE LAW FIG. 6

AMP.

3,461,453 REDUCHJG-NOISE WITH DUAL-MODE ANTENNA Tingye Li, Middletown, N..I., assignor to Bell Telephone Laboratories, Incorporated, Murray Hill and Berkeley Heights, N.J., a corporation of New York Filed Aug. 30, 1967, Ser. No. 664,385 Int. Cl. Htl4b 7/02 US. Cl. 343-100 4 Claims ABSTRACT OF THE DISCLOSURE A dual-mode antenna radiates a primary and auxiliary lobe both effectively intercepting the same region of space in the near-field zone of the antenna but intercepting mutually exclusive regions of space in the far-field zone. Both lobes receive components of noise originating in the near-field zone but only the primary lobe receives the desired signal in the far-field zone. The received power in the auxiliary lobe is subtracted from that in the primary lobe to reduce the noise fluctuations which are common to both lobes.

BACKGROUND OF THE INVENTION This invention relates to the reduction of noise fluctuations in received signals from distant radio sources and, more particularly, to the reduction of effects of atmospheric noise in radio astronomy systems.

Faint radio sources in deep space are presently subjects of substantial scientific investigation. Reception of useful signals from these sources cannot be successfully obtained by simply aiming an antenna toward the source because undesired noise is received together with the desired signal which is usually of relatively low power, and the noise causes fluctuations of a significant magnitude so that the desired signal is almost entirely obscured. A principal source of these fluctuations is the atmospheric noise in the near-field zone of the antenna.

A method of reducing the eifect of this atmospheric noise has been presented by R. G. Conway in Nature, volume 199, page 1177, 1963, and elaborated upon by I. W. M. Baars in Nature, volume 212, page 494, 1966. This technique employs a parabolic dish antenna having two separate feeds: an on-axis feed produces a beam which receives both the desired signal and the undesired atmospheric noise originating in the near-field zone of the antenna; an off-axis feed produces a beam which points significantly away from the on-axis beam and hence, does not receive the desired signal in the far-field zone of the antenna but does receive the near-field zone noise. A differential comparison of the power in the two beams reduces the noise fluctuations which were superimposed upon the desired signal received in the on-axis beam.

In accordance with the invention the Conway-Baars noise-reference technique has been modified and improved. The prior method is useful only Where two independent feeds can be employed; a horn-reflector antenna which is superior to the parabolic dish in regard to inherent noise characteristics cannot be adapted to two independent feeds to be placed at the throat of the horn without incurring a loss of discrimination between the signals associated with each feed.

Summary of the invention In accordance with the invention the reference-noise technique is applied to the superior horn-reflector antenna by substituting two modes of propagation for the two independent feeds employed in the prior art. Horn-reflector antennas are capable of supporting multiple modes of propagation while maintaining discrimination. The anl ite States Patent tenna must be designed to support two modes, the first of which produces a lobe which intercepts the signal and the atmospheric noise, and the second of which produces a lobe which intercepts the same atmospheric noise but diverges from the first lobe in such a manner that it intercepts no radiation in the region of the desired signal.

The boundary region between the near-field and farfield zones of the antenna is defined as the region in which D/k is on the order of unity times d, the distance from the antenna aperture to the boundary of the region, where D is the antenna aperture diameter and A is the wavelength corresponding to the frequency of the desired signal. The antenna must be designed so that the near-field zone, where both lobes intercept the same effective region, extends far enough into space to encompass the entire extent of the atmospheric noise, but only so far that the first lobe alone intercepts the desired signal.

The two modes are separated and the difference in power is recorded in a manner well known in the art. The comparison is made on a time average basis, not instantaneously. Each of the receiving modes receives only its coresponding component of the noise, and the variations of each component at a given instant are independent. An instantaneous comparison would therefore not reduce the noise since the variations in each component are not synchronized, but because the noise is random over a period of time it can be assumed that the average fluctuations in each component are similar and the average power in each component is effectively equal. A differential comparison of this quantity can thus be made to reduce efiects of the noise commonly received in the two modes.

Description of the drawings The novel features of the invention will become apparent from the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of the apparatus and a representation of the antenna patterns;

FIG. 2 is a cross section of the antenna pattern in the far-field zone;

FIG. 3 is a cross section of the antenna pattern in the near-field zone;

FIG. 4 is a diagram of the field configuration of the TE mode in a circular waveguide;

FIG. 5 is a diagram of the field configuration of the TM mode in a circular waveguide;

FIG. 6 is a diagram partially in block form and partially schematic showing the comparator of the invention.

Detailed description Referring to FIG. 1, there is shown, by way of example, a system for receiving in the far-field zone of antenna 15 a weak signal 10, assumed to be from a distant star, and eliminating from the reception the effects of noise originating in the near-field zone. Horn-reflector antenna 15 is shown radiating two antenna lobes 11 and 12 corresponding, respectively, to the two modes T13 and TM of the circular waveguide feeding the antenna. In the figure the effective boundaries of the lobes are represented by two different broken lines in a plane view; in space the lobes are figures of revolution.

In the far-field zone of the antenna the TE mode lobe 11 encompasses effectively a conical volume rotated about the axis of propagation 19 of the horn-reflector antenna 15. In the same region the TM mode lobe 12 encompasses effectively a hollow conical volume rotated about the axis 19, the lobe being substantially exterior to the TE mode lobe 11. A cross section is illustrated in FIG. 2 where it is shown that at any given point in the far- 3 field zone the two lobes are defined by concentric circles such that the T M mode lobe 12 forms an efiective doughnut about the TE mode lobe 11.

The portions of lobes 11 and 12 in the near-field zone of the antenna both eifectively encompass a single cylindrical volume 14. Lobe 12 contains an axial hole 13 in its interior which, in the near-field zone, is relatively narrow and can be disregarded in the operation of the apparatus. The cross section of the lobes in the nearfield zone shown in FIG. 3 further explains this geometry.

Assuming that the only undesired noise originates in the atmosphere 18 and the antenna 15 is designed so that the near-field zone extends beyond the atmosphere 18, and that the antenna 15 is aligned so that the desired signal is intercepted by the TE mode lobe 11 in the far-field zone of the antenna, the signal 10 will be received by antenna in the TE mode. Also received in the TE mode will be the noise intercepted by lobe 11 in the near field zone. The TM mode lobe 12 intercepts no signal in the far-field zone but intercepts the atmospheric noise in the near-field zone.

The TE mode containing the received signal 10 and atmospheric noise is reflected by reflector 16 and guided down the horn 17 into a circular waveguide 21. The TM mode containing atmospheric noise is likewise conducted to waveguide 21.

The two modes are simultaneously conducted in waveguide 21 as is shown by the diagrams for the field configuration of the TE and TM modes, respectively, in FIGS. 4 and 5. The two modes each contain a component of the random atmospheric noise; the noise fluctuations in the two modes are not instantaneously identical, but the average values over a relatively long period are expected to be the same.

The two modes are separated by a mode selective coupler 31 which picks off the energy in the waveguide at opposite ends of a diameter and combines these signals in phase to produce the dominant TE mode, and combines the signals out of phase to produce the auxiliary TM mode. Such a device is described in the Bell System Technical Journal, Volume 42, pages 1301-1304 (J. S. Cook and -R. Lowell, The Autotrack System, 1963.) The TE and TM modes thus separated are conducted by coaxial cables 32 and 33, respectively, to comparator 40. A suitable example of such a comparator is the Dicke radiometer disclosed in Radio Astronomy, J. D. Kraus, pages 248 and following, and which is reproduced as FIG. 6.

Referring to FIG. 6, the voltage signals on cables 32 and 33 are connected alternately to point 54 through switch 41. Switch 41 is triggered by a switch generator 49 such that terminals 22 and 23 of cables 32 and 33, respectively, are each connected for half of the time. The signals thus alternately appearing at point 54 are amplified by amplifier 42. The amplified signal 50 enters a square law detector 43 and a signal 51 proportional to the power is produced. The signal 51 is fed into a multiplier-integrator 45, wherein the signal 51 is transferred to point 24 by a mutually inductive coupling 47 and a negative of the signal 51 is simultaneously produced by the coupling 47 at point 25.

Switch 46 triggered by switch generator 49 is alternately connected to points 24 and 25 in such a manner that the signal appearing at point 24 and corresponding positively to the TE mode signal at terminal 22 will be connected to point 55 by switch 46 during one half cycle. The signal appearing at point 25 and corresponding negatively to the TM mode signal at terminal 23 will be connected to point 55 by switch 46 during the other half cycle.

The signal at point 55 is integrated over a period, thus producing a signal proportional to the difference of the power of the signals at points 22 and 23. The output is the result-ant of the differential comparison of the power of the signals on the TB and TM modes thus representing the desired signal 10 without the impingent fluctuations produced by the atmospheric noise in region 18. The output is fed to a recorder not shown.

While the principles of the invention have been described in connection with a specific embodiment, it is to be clearly understood that this description as made only by way of example, and not as a limitation upon the scope of the invention as set forth in the accompanying claims.

What is claimed is:

1. Radio astronomy apparatus having an antenna operating in a first and second channel, said first channel receiving a desired signal and undesired noise and said second channel receiving only said undesired noise, and means for comparing said both channels so as to reduce the effects of said undesired noise characterized in that said channels are formed, respectively, by first and second coaxial electromagnetic modes said first and second modes being coincident from said antenna through the atmosphere and capable of receiving in common atmospheric noise and said first mode being capable of receiving exclusive of said second mode said desired signal from a region substantially beyond the atmosphere.

2. Apparatus for receiving an electromagnetic signal from a source beyond the atmosphere and removing from said received signal the elTects of undesired atmospheric noise comprising a dual mode antenna having first and second modes producing, respectively, a first axially directed receiving lobe being directed toward said source and a second axially directed receiving lobe at least a portion of which is mutually exclusive of said first lobe and at least a portion of which is coincident with said first lobe, said antennas dimensions being such that said coincident portion extends through the atmosphere, said both lobes being oriented to receive in said coincident portion their respective components of said undesired atmospheric noise, means for separating said two modes, and means for differentially combining on a time average basis the signals on said two modes and producing a resultant signal proportional to the power of said electromagnetic signal substantially exclusive of said atmospheric noise.

3. Apparatus for receiving a low level electromagnetic signal originating beyond the atmosphere comprising a dual mode antenna having first and second coaxial modes, said antenna having a near-field zone which extends beyond the atmosphere and a far-field zone exclusive of said near-field zone which extends from the atmosphere and beyond, said first mode intercepting electromagnetic radiation along a beam about its axis in said nearand farfield zones and said second mode intercepting electromagnetic radiation along a beam similar to said first beam in said near-field zone and intercepting electromagnetic radiation exclusive of the radiation intercepted by said first mode in said far-field zone such that said second mode intercepts exclusively atmospheric noise in said near-field zone, means for separating said two modes, means for combining differentially the time average reception of said two modes so as to produce an output representative of said low level signal exclusive of said atmospheric noise received by said both modes in common in said near-field zone.

4. Apparatus for receiving a low level electromagnetic signal originating beyond the atmosphere comprising a dual mode antenna having first and second coaxial modes, said antenna having a near-field zone which extends beyond the atmosphere and a far-field zone exclusive of said near-field zone which extends from the atmosphere and beyond, said first mode intercepting electromagnetic radiation along a beam about its axis in said nearand far-field zones and said second mode intercepting electromagnetic radiation along a beam similar to said first beam in said near-field zone and intercepting electromagnetic radiation exclusive of the radiation intercepted by said first mode in said far-field zone such that said second mode intercepts exclusively atmospheric noise 5 6 in said near-field zone, means for separating said two References Cited modes, means for alternately sampling the reception on UNITED STATES PATENTS said first mode and the reception on said second mode, means for squaring the sampled reception on said two 7/1961 Wllcox 325*475 X modes, and means for reversing the polarity of said second 5 2 i 2 a a '1 mode and balancing the squared reception of said first and 1 9 8 Gian omemco second modes against one another over a period of time RODNEY D. BENNETT, JR., Primary Examiner to produce an output, whereby said output is representative of said low level signal exclusive of said atmospheric TUBBESING Asslstant Exammer noise received by said both modes in common in said 10 US, Cl. X.R. near-field Zone. 325-476 

